A conventional photoconductive electrostatic master comprises a sheet of electrically conductive paper having a surface coated with a layer of finely divided zinc oxide in a dielectric resin binder. This is called the photoconductive layer and is usually applied from a solvent such as toluene. The zinc oxide is non-conductive in the dark, and this property, plus the dielectric property of the resin binder, renders the conductive layer capable of receiving a uniform electrostatic charge and retaining it for an appreciable period of time in darkness. However, when the zinc oxide is subsequently exposed to light, around an image, the zinc oxide becomes conductive and the electrostatic charge in the non-imaged areas is discharged through the paper. The remaining charged area, that corresponds to the original image, is then made visible by contacting the surface carrying the electrostatic charged pattern with finely divided developer material, referred to as toner, bearing a charge opposite to that of the electrostatic image. The developer material deposits on the plate in the image configuration and is fused to the plate surface by heat. Alternative methods of fusing involve liquid toner or pressure fusing technology. The electrostatic master is then treated with an aqueous etch solution which renders the non-imaged areas hydrophilic. The toner used in the imaged area is hydrophobic and oleophilic.
In use in printing, the electrostatic masters are wet with an aqueous fountain solution which functions to wet the hydrophilic non-imaged areas exclusive of the oleophilic imaged area. When the plate is contacted with an ink roll, coated with an oil based printing ink, the ink is repelled by the hydrophilic surface in the non-imaged areas and deposits only on the oleophilic imaged area of the plate. The ink then may be transferred to a paper sheet to provide the final product.
The plate will also function when the oil and water are applied simultaneously.
In practice, the fountain solution and ink are applied to the master once for each printed sheet produced. In long run masters, it is necessary to protect the paper base, which preferably is a wet strength paper, against the action of the aqueous fountain solution. This protection is usually achieved by providing the paper base with one or more water resistant layers, which can be referred to as barrier coatings, between the base and the photoconductive layer.
Absorption of the fountain solution into the paper base has several undesirable effects. Primarily, it causes a dimensional expansion of fibers in the paper in turn causing distortion of the paper and image resulting in imperfect registration and wrinkling of the plate. Attack by water on the zinc oxide or photoconductive layer bond as well as on various sub-coatings reduces the strength of these bonds. Eventually picking of portions of the zinc oxide coating and/or sub-coatings off the master onto the printing blanket causes loss of image continuity and/or background fill in.
Conventional barrier coatings, used in the construction of lithographic paper masters, are described in U.S. Pat. Nos. 3,298,831, issued to Lau; 3,653,894, issued to Levy; 3,839,033, issued to Matsuno; and 3,787,235, issued to Honjo. The use of natural and synthetic adhesives to provide surface strength and water resistance to lithographic masters are summarized in TAPPI monographs numbers 36 and 37, by the Technical Association of the Pulp and Paper Industry (TAPPI), One Dunwoody Park, Atlanta, Georgia 30341.